I actually think that if anything, signing had the opposite effect, by showing the value of communication before children are ready to speak intelligibly. Exposure to multiple spoken languages on the other hand... I can see how that would cause a delay, but I still think the advantages outweigh the disadvantages.
Children raised in bilingual environments from birth typically develop speech slower than their peers in single-language environments, because they're dealing with the difficulties of learning two distinctly different sets of linguistics. But once they learn to communicate effectively, they usually catch and then exceed their single-language peers, and it usually happens around ages 4-5... just in time for school.
This is borne out by research, and I've seen it in my wife's family as well.
As far as TV, I remember reading a study that purported that children who watched educational shows like Dora and Blues Clues had bigger vocabularies at a young age than kids who didn't. By contrast, children who watched non-educational TV had smaller vocabularies.
I'm not surprised. If the kids are spending lots of face time with Mommy and Daddy, they still only have those two sources for language development. Even the most well-spoken of us have our own habits we use a little too much, couples will tend to fall into patterns together, and the everyday experience of living together as a family means certain conversations happen over and over and over and over and...
Television is great tool for expanding a child's experiences of linguistic patterns. And that's just one advantage.
As with most things, TV is merely a tool, neither good nor bad. It's all in how you use it.
Not a TV anecdote, but a "hearing things from a different source" anecdote... my daughter managed to come up with a word for all her basic needs (drink, change, nap, etc.) by six months except for one: food. A couple months later, my wife enrolled in cooking school, and my mom was babysitting. Shortly thereafter, DD started saying, "num-num" to let us know she was hungry. Wondering where it came from, I asked my mom, who said she was constantly saying, "Mmmmm, num-num" between every spoonful during feedings. It never occurred to DW or myself to say any such thing, but my mom did it reflexively. I don't think I need to mention how much more peaceful the house becomes when the baby can ask for what she needs, and that was the last piece of the puzzle, so we were very grateful.
It would seem that a child who spends more time with an adult than a TV would have a greater opportunity to ask questions, but questions usually don't come until well after 13 months. In the early stages children learn through observation. If the TV is offering more educational content than what can otherwise be provided, then it seems to me it's a positive thing. But that's coming from someone who uses TV as a last resort.
It doesn't have to be a binary proposition, adult or TV. In a perfect world, the two are offered together... the parent watches the show with the child, reacts appropriately, comments on certain aspects of it, and answers any questions as they come up.
